Methods of producing silicon of high purity



METHODS OF PRODUCING SILICON OF HIGH PURITY `Original Filed Aug. 4, 1956Inventor J M.WILSON A ltorn ey United States Patent @Hice A 3,053,631 3Patented Sept. 1l, 19.62

3,053,631 METHODS F PRODUCIN G SILICON 0F HIGH PURITY Jack McCreathWilson, London, England, assignor to International Standard ElectricCorporation, New York, N.Y., a corporation of Delaware Originalapplication Aug. 4, 1956, Ser. No. 604,001, now Patent No. 2,888,328,dated May 26, 1959. Divided and this application May 8, 1959, Ser. No.811,982 Claims priority, application Great Britain Aug. 16, 1955 4Claims. (Cl. 23-Z04) I'he invention relates to a process for themanufacture of pure silicon, and particularly to a process to excludethe impurity boron from the silicon. This application is a division ofU.S. application Serial No. 604,001, tiled August 4, 1956, now U.S.Patent 2,888,328, issued May 26, 1959.

Boron is electrically one of the most important impurities in siliconand it cannot be removed from solid silicon by such known purificationprocesses as zone refining and repeated crystal pulling. In BritishPatent No. 745,- 698 (I. M. Wilson et al.) a method of production ofvery pure silicon has been described which depends upon thedecomposition by heat of silane (SiH4), the silane being generated bythe chemical reaction of silicon tetrachloride and lithium aluminiumhydride. Should the silicon tetrachloride (or the lithium aluminiumhydride or the solvent for either of these) contain any boron compoundsas impurities, which is often the case, diborane could be produced.Diborane, however, is a volatile hydride of boron and decomposesthermally to form boron and hydrogen. Thus when silane containingdiborane is thermally decomposed so is the diborane and boron appears asan impurity in the silicon. In the process described in the parentspecication, predecomposition puriiication of the silane Was carried outat a temperature below its cracking temperature, but it has been `foundthat this is not adequate to remove boron completely.

It is accordingly the object ofthe present invention completely toeliminate the presence of diborane in the silane.

In order to clearly understand the principle of the invention it isnecessary firstly to consider the method of preparation of diboraneconsisting of reacting boron tricbloride with lithium aluminium hydride.

It has been shown by several workers that the order of addition of thereagents determines the course of the reaction. Thus when an etherealslurry of lithium aluminium hydride is added to a solution of borontricbloride in ether a steady evolution of diborane occurs according tothe equation When the reagents are addd in the reverse order and borontrichloride is added to lithium aluminium hydride, a negligible amountof diborane is generated until 50% of the stoichiometric amount of borontricbloride has been added. The explanation is that in this case thereis a stepwise reaction, lithium borohydride being formed as anintermediate product. No diborane is formed until the whole of thelithium aluminium hydride has been converted to borohydride.

In accordance with the present invention, in the manufacture of siliconby the reaction of a solution of silicon tetrachloride with a liquidsuspension of lithium aluminium hydride to produce silane and thesubsequent thermal decomposition of the silane, the said reaction iscarried out by adding a solution of silicon tetrachloride to the lithiumaluminium hydride and maintaining an excess of lithium aluminiumhydride, whereby the amount of diborane produced from impurities in thereagents is kept to a minimum.

An embodiment of the invention will now be described with reference tothe accompanying drawing which shows diagrammatically an apparatus forcarrying out the invention.

A suspension of lithium aluminium hydride in tetrahydrofuran is preparedin vessel 1 and tap funnel 10, and of silicon tetrachloride intetrahydrofuran in the upper part of tap funnel 9*.

A stream of inert gas such as argon, nitrogen, or hydrogen is passedinto vessel 1 from an inlet pipeZ through tap 3, and through the lowerpart of the funnel 5. 'This sweeps all the air out of vessel 1. After aninterval a quantity of lithium aluminium hydride solution contained intap funnel 10 is added to that already contained in vessel 1 by theopening of tap 8 which allows the hydride to ow through a tube 12 packedwith material to give large surface exposure. Tap 8 is then closed andtap 7 opened to allow an equivalent stoichiometric amount of silicontetrachloride to flow into vessel 1 from the upper half of tap funnel 9.The process of alternately opening taps 7 and 8 to allow equivalentstoichiometric amounts of lithium aluminium hydride and silicontetrachloride to pass into Vessel 1 is continued. The silane reactionthus takes place in an excess of lithium aluminium hydride and thesubstantially pure silane thus generated.

-Any silicon tetrachloride, which may contain boron tricbloride, whichmay also become entrained in the gas stream, is removed by passing thegas stream through the packed tube 12 down which the solution of lithiumaluminium hydride flows. This process also removes any diborane whichmay have been formed, in parts of the apparatus between the generatorflask and the packed column, by reaction between any lithium aluminiumhydrid'e and silicon tetrachloride, containing boron compounds asimpurities, which have become entrained in the gas stream. This processcould 'also be used if :for any reason the lirst stage of the processwas not carried out. After passing through condensers, etc. whichprevent any of the reactants passing along with the gas, the gas stream,now consisting only of the inert gas and silane, is then passed via pipe13 to the apparatus wherein the silane is decomposed.

It is to be understood that the apparatus is provided With all thenecessary trap joints between the different portions thereof. All theenclosing walls and pipes shown should be made of high purity silica.The silica of which vessel 1 in particular is made, and' preferably alsoother vessels and tubes, should be free from any boron or other materialwhich may form a significant impurity in the silicon.

By preparing the silane as described above, the resulting silicon had aboron content below the detection limit of the analytical method (i.e.below 0.2 p.p.m.).

It is to be understood that there may be interposed between the tube 13and the vessel in which the silane is thermally decomposed, a Vesselheated to a temperature lower than the decomposition temperature ofsilane but high enough to decompose completely other hydrides which mayexist as impurities in the silane, and perhaps any small traces ofborane which may remain.

It may be noted that the use of tetrahydrofuran for forming solutions orsuspensions of the reactants is preferable to the use of ether becauseof the higher flash point of the former. This has been found to reducethe chances of explosion in the plant used. This plant has beenconsidered to be extremely dangerous when ether has been used.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is by wayof example and not as a limitation on the scope of the invention.

What is claimed is:

l. A process of manufacture of substantially pure silane by the reactionof silicon tetrachloride having boron present as au impurity, withlithium aluminium hydride to produce silane, comprising carrying outsaid reaction by adding said silicon tetrachloride to a quantity oflithium aluminum hydride to produce silane gas having as an impuritysome diborane gas as a resulting reaction product, and passing thesilane produced by said reaction through a passage filled with packingholding a liquid suspension of lithium aluminium hydride to removediborane present in the silane.

2. A process as claimed in claim l, in which said liquid suspension oflithium aluminium hydride is a suspension in tetrahydrofuran.

3. A process of manufacture according to claim l, further comprisingadding lithium aluminium hydride, said quantity to maintain an excess ofsaid lithium aluminium hydride during said reaction to minimise theproduction of diborane.

4. A process of manufacture of silane substantially free from diboraneas an impurity, by the reaction of silicon tetrachloride having boronpresent as an impurity,

with lithium aluminium hydrid'e, comprising adding said silicontetrachloride to a quantity of lithium aluminium hydride in a suspensionin tetrahydrofuran, said suspension having an excess of saidY lithiumaluminium hydride, to produce silane gas having as a resulting reactionproduct some diborane gas, passing said produced gases through a passagefilled with packing, and adding a suspension of lithium aluminiumhydride to said quantity through the packing in said passage, wherebydiborane gas is removed in passing through said passage and excess oflithium aluminium hydride is maintained in said quantity.

References Cited in the file of this patent UNITED STATES PATENTS2,805,982 Swinehart et al. Sept. l0, 1957 2,970,040 Conn Jan. 31, 19612,987,139 Bush June 6, 1961 FOREIGN PATENTS 745,698 Great Britain Feb.29, 1956 OTHER REFERENCES Wartik et al.: Journal of the AmericanChemical Society, vol. 75, p. 836 (1953).

4. A PROCESS OF MANUFACTURE OF SILANE SUBSTANTIALLY FREE FROM DIBORANEAS AN IMPURITY, BY THE REACTION OF SILICON TETRACHLOIDE HAVING BORONPRESENT AS AN IMPURITY, WITH LITHIUM ALUMINIUM HYDRIDE, COMPRISINGADDING SAID SILICON TETRACHLORIDE TO A QUANTITY OF LITHIUM ALUMINIUMHYDRIDE IN A SUSPENSION IN TETRAHYDROFURAN, SAID SUSPENSION HAVING ANEXCESS OF SAID LITHIUM ALUMINIUM HYDRIDE, TO PRODUCE SILANE GAS HAVINGAS A RESULTING REACTION PRODUCT SOME DIBORANE GAS, PASSING SAIDPRODUCTED GASES THROUGH A PASSAGE FILLED WITH PACKING, AND ADDING ASUSPENSION OF LITHIUM ALUMINIUM HYDRIDE TO SAID QUANTITY THROUGH THEPACKING IN SAID PASSAGE, WHEREBY DIBORANE GAS IS REMOVED IN PASSINGTHROUGH SAID PASSAGE AND EXCESS OF LITHIUM ALUMINIUM HYDRIDE ISMAINTAINED IN SAID QUANTITY.